Gain matching in a biamped system.

Depends on your tolerance for the difference in respective sound levels. Electronically it makes no difference. Personally I wouldn't settle for less than a perfect match, but some folks might like a bump up in the lower end of 1 or 2 db's.

The two amplifiers, I suppose they are different, should not differ too much in their sonic characteristics.
To match the gain, use a sound level meter and fix an attenuator to correct level from the more efficient amp.